This invention relates to an improved intraocular lens for implantation in the eye following removal of the natural lens during cataract surgery. More specifically, it relates to a compression molding process for manufacturing an intraocular lens in which the supporting haptics exhibit an increased resistance to breakage during manipulation.
The filamentary haptic of an intraocular lens is designed to facilitate insertion of the lens into the eye and to provide stable fixation of the implanted lens to prevent the lens from decentering. The filamentary haptic is attached to and extends outwardly from the periphery of the optical lens body. Most intraocular lenses have two haptics displayed at positions 180.degree. apart from each other on the optic lens body.
Intraocular lenses are typically of two types: the threepiece lens where two haptics are mechanically fixed to the lens optic, and the one-piece lens where the haptics and optic are made as a single unit.
It is critical that the haptic of the lens exhibits significant resistance to breakage during use. Although certain haptic materials such as polypropylene used in construction of a three-piece lens offer acceptable resistance to breakage, other conventionally used haptic materials such as polymethyl methacrylate (PMMA) are brittle and frequently prone to breakage. This problem becomes especially acute when the haptics are lathe cut from a single lens blank to prepare a one-piece lens with integrally attached haptics. The problem of haptic breakage is a serious one, and efforts have been made to provide the haptics with an increased resistance to breakage.
Several thermoplastic forming techniques have been used to induce orientation in the material to impart enhanced mechanical properties in PMMA intraocular lens haptics. One such effort is disclosed in U.S. Pat. 5,169,569. This patent describes preparing intraocular lenses with haptics exhibiting greater ductility and fatigue resistance. The PMMA sheet plastic is modified by means of blowing the sheet into a hemispherical bubble followed by forming into a flattened circular portion.
A second effort is disclosed in European Patent Application 0,438,043 A2. The PMMA material is modified by multi-axis stretching to produce haptics with increased tensile strength, flexibility and resiliency.
Another previous method of making intraocular lenses was through a compression molding process which introduced deformation, and thus material orientation, throughout the entire lens. This type of lens demonstrated enhanced haptic mechanical property performance against breakage, but the particular method shows a less desirable Nd:YAG laser interaction characteristic.
The Nd:YAG laser is used for treatment of posterior capsular opacification. In this treatment the posterior capsule is ruptured after being hit with a focused laser beam. The rupture typically leaves a hole 2 mm-4 mm in diameter through which light can pass. Posterior capsular opacification usually occurs several months after intraocular lens implantation. During the laser procedure, the intraocular lens is frequently hit by the beam. This hit, depending upon the energy of the burst and the location of the hit, can cause varying degrees of damage to the lens. It has been shown in our work that the laser energy interacts differently with oriented and nonoriented materials. In the nonoriented material, referred to as conventional material, the laser energy is dissipated along the three axes of the lens. This typically results in distinct cracks which may propagate a significant distance through the thickness dimension of the lens. The oriented material typically retards crack propagation through the thickness dimension and therefore demonstrates more extensive lateral cracking.
While this is not a vision compromising condition, it is undesirable because it results in light scattering from oblique illumination making eye examinations considerably more difficult. The optic material described in this invention is oriented, but the degree of orientation is balanced such that the resulting crack propagation behavior is similar to the nonoriented material.
A method used to produce a one-piece intraocular lens having a low degree of orientation in the optic region is by injection molding. This previously employed method resulted in an optic region with acceptable characteristics under YAG laser irradiation, but the haptic region was also insufficiently oriented, resulting in an unacceptably high fracture rate.
It is, therefore, an object of the present invention to provide an improved intraocular lens wherein the haptics exhibit enhanced mechanical properties and resistance to breakage while the optics exhibit the conventional Nd:YAG laser interaction characteristics, and a process for making such a lens.
It is a further object of the invention to provide a one-piece PMMA intraocular lens made by the compression molding process and exhibiting the above properties.